Various types of electronic semiconductor devices employ capacitive structures to effect proper circuit operation. Examples of such devices include, among others, insulated-gate field-effect transistors (IGFETs), insulated-gate thyristors, discrete capacitors and various types of optics devices. In the commonly-used IGFET, for example, a gate controls an underlying surface channel joining a source and a drain. The channel, source and drain are typically located in a semiconductor substrate material, with the source and drain being doped oppositely to the substrate material and located on either side of the channel. The gate is separated from the semiconductor substrate material by a thin insulating layer such as a gate oxide having a substantially uniform thickness. To operate the IGFET, an input voltage is applied to its gate and, through the capacitive structure defined by the electrode material on either side of the gate oxide, this input voltage causes a transverse electric field in the channel. This field then modulates the longitudinal conductance of the channel to electrically couple source and drain regions.
Various benefits can be realized by reducing the dimensions of such electronic semiconductor devices. One benefit is the ability to increase the number of individual devices that can be placed onto a single silicon chip or die without increasing its relative size. Also, increasing the number of individual devices, especially IGFETs, leads to increased functionality. Yet another benefit is increased speed of the individual devices as well as their collective arrangements.
For decades now, the semiconductor industry has been realizing these size-reduction benefits using silicon substrates at a tremendous rate, as exemplified by the electrical performance of MOS-type (metal-oxide-semiconductor) silicon-based IGFETs doubling every 2 to 3 years. However, the International Technology Roadmap for Semiconductors (ITRS) notes that “traditional scaling” of such silicon-based IGFETs (e.g., planar bulk Si-MOS structures) is beginning to face limits to this continued progress. The extent to which the semiconductor industry can drive this scaling of silicon-based IGFET devices is unknown, but there is agreement that the current rate of technology evolution permits only about 4 more technological-advancement nodes of this “classical” silicon-based approach.
Scaling to 4 more technology nodes would lead to effective feature sizes of approximately 20-30 nm. However, achieving even this objective would require significant technological breakthroughs. Beyond this point, there is generally industry-wide agreement that traditional silicon-based IGFET technology would likely have to be replaced by future innovations, including new materials and devices. As such, an entirely new and different era and area of technology would have to be introduced.
One such very promising material is Germanium (Ge) because of its very high carrier mobility. In the past few decades, researchers have been trying to build MOS-type transistors and capacitors using Germanium (Ge) and silicon-Germanium (SiGe) for integrated electronic and/or optical circuit applications, due to some of its superior qualities to silicon (Si). However, various problems with Ge or SiGe have frustrated these efforts. For example, the lack of a sufficiently stable native oxide for the Ge has undermined the ability to passivate the Ge or SiGe surface and form a gate dielectric material for MOS-type devices.